<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        li {
            list-style-type: none;
        }
        a {
            text-decoration: none;
            font-size: 14px;
        }
        .nav {
            width: 341px;
            height: 50px;
            margin: 100px auto;
        }
        .nav > li{
            position: relative;
            float: left;
            width: 80px;
            height: 41px;
            text-align: center;
        }
        .nav li a{
            display: block;
            width: 100%;
            height: 100%;
            line-height: 40px;
            color: #333;
        }
        .nav>li>a:hover {
            background-color: #eee;
        }
        .nav ul{
            display: none;
            position: absolute;
            top: 41px;
            left: 0;
            width: 100%;
            border-left: 1px solid #FECC5B;
            border-right: 1px solid #FECC5B;
        }
        .nav ul li{
            border-bottom: 1px solid #FECC5B;
        }
        .nav ul li a:hover{
            background-color: #FFF5DA;
        }
    </style>

</head>
<body>
    <ul class = "nav">
        <li>
            <a href = "#">微博</a>
            <ul>
                <li>
                    <a href = "">私信</a>
                </li>
                <li>
                    <a href = "">评论</a>
                </li>
                <li>
                    <a href = "">@我</a>
                </li>
            </ul>
        </li>
        <li>
            <a href = "#">微博</a>
            <ul>
                <li>
                    <a href = "">私信</a>
                </li>
                <li>
                    <a href = "">评论</a>
                </li>
                <li>
                    <a href = "">@我</a>
                </li>
            </ul>
        </li>
        <li>
            <a href = "#">微博</a>
            <ul>
                <li>
                    <a href = "">私信</a>
                </li>
                <li>
                    <a href = "">评论</a>
                </li>
                <li>
                    <a href = "">@我</a>
                </li>
            </ul>
        </li>
        <li>
            <a href = "#">微博</a>
            <ul>
                <li>
                    <a href = "">私信</a>
                </li>
                <li>
                    <a href = "">评论</a>
                </li>
                <li>
                    <a href = "">@我</a>
                </li>
            </ul>
        </li>
    </ul>
    <script>
        //1.获取元素
        var nav = document.querySelector('.nav');
        var lis = nav.children;
        for(var i = 0; i < lis.length; i++){
            lis[i].onmouseover = function(){
                this.children[1].style.display = 'block';
            }
            lis[i].onmouseout = function(){
                this.children[1].style.display = 'none';
            }
        }
    </script>
</body>
</html>